1. Field of the Invention
The field of the invention is that of transmitting data frames within communication networks, more particularly transmission involving compression/decompression.
2. Description of the Prior Art
To increase their connection capacities, some communication networks comprise compressors/decompressors for compressing data contained in frames to be transmitted.
In such networks, compression/decompression is referred to as “layer 1 compression/decompression” because it is applied to the physical layer of all frames to be compressed, regardless of their type. Layer 1 compression consists in detecting redundancies between the data contained in identical portions comprising stream frames of frames to be transmitted (known as input frames) in order to eliminate them but to report them so that they may be reconstituted on decompression. Stream frames compressed in this way are stored temporarily in buffer queues and routed to their destination as a function of available transmission capacity.
An input frame is a frame that feeds the compressor. It is made up of a multiplicity of stream frames constituting initial frame portions associated with different transmission channels. For example, in TDMA transmission mode, each input frame comprises a fixed number of time slots each comprising one byte. According to the ITU-T G.703/G.704 recommendations, this fixed number is equal to 32 in the case of E1 type frames and to 24 in the case of T1 type frames. The frame frequency is generally 8 kHz, enabling an E1 frame to convey 31 usable channels at 64 kbps (kbits/s), with one channel per time slot.
In a cellular network, an A-bis interface couples a base station controller (BSC) and one or more base transceiver stations (BTS) using E1 or T1 type frames. The A-bis interface conveys two types of stream: traffic streams (speech or data) and signaling streams. To convey traffic streams, each byte of the E1 or T1 frame is divided either into four “nibbles” each carrying one 16 kbps traffic channel (this is known as the “full rate” (FR) mode) or into eight “half rate” (HR) traffic channels, in which each bit corresponds to a voice channel. The signaling streams are generally carried by 2-bit (16 kbps) channels of 8-bit (64 kbps) channels. An initial E1 or T1 frame therefore constitutes a set of transmission channels with a size running from 1 bit for 8 kbps channels to 8 bits for 64 kbps channels; each channel conveys stream frames specific to each transmission channel.
Traffic streams are conveyed by transcoder/rate adapter unit (TRAU) frames exchanged between each BTS and the mobile switching center of the public land mobile network (PLMN), via a transcoder/rate adapter unit (TRAU), which in particular converts 13 kbps compressed speech data into 64 kbps digitized speech data to make the speech channels compatible with the mobile switching center. The mobile switching center and the TRAU are coupled via an A interface and the BSC and the TRAU are coupled via an A-ter interface.
Hereinafter, the expression “active channel” means any traffic channel that has been set up, i.e. in respect of which a call setup procedure has been executed correctly using a signaling channel, this procedure leading to the provision of a traffic channel that is dedicated to the call concerned and is maintained until the end of the calling phase. A traffic channel is used to convey TRAU frames exchanged between a calling party and a called party throughout the duration of the call, including during phases of silence.
The TRAU frames are of four types: speech (or data transfer) frames, which contain at least control data and payload data, silence frames, which contain at least control data, silence descriptor (SID) frames, which contain at least control data and payload data, and bad frames, which contain at least control data.
Hereinafter, the use of TRAU frames to transfer data will be lumped together with the use of TRAU frames to transport speech, exactly the same processing being applied in both cases.
When the decompressor receives compressed stream frames, it decompresses them successively to recompose (or restore) the original frames of which they constitute portions. This compression/decompression transmission mode may be described as asynchronous because it introduces “end-to-end” transmission delays for the recomposed original frames (or streams) which vary as a function of the load on the network. Moreover, these delays may vary considerably from one transmission channel to another. Also, the maximum transmission delay, which may be considered as guaranteed by the network operator, is high because the processing carried out along the whole of the transmission path is non-deterministic.
The non-deterministic nature of the end-to-end delay, combined with the fact that appropriate margins must be allowed to guarantee a maximum transmission delay, make it virtually impossible to use the compression technique described hereinabove in a voice communication network such as a GSM network, for example.
Thus one object of the invention is to improve on the above situation, and in particular to guarantee an end-to-end delay over the transmission path, as well as achieving an optimum compromise between the compression gain and the end-to-end delay, and where possible achieving compatibility with operation using layer 1 compression/decompression.